In a tennis racquet, the vibration of the impact when the ball is hit is transmitted to the frame from the gut plane (strings) and is further transmitted to the body of the player, i.e., wrist, elbow, etc., from the grip portion via the handle portion. If the body is frequently subjected to such impact vibration over a long period of time, cases of injuries such as peritendinitis and tennis elbow are commonly seen. From the incidence rate as well, this is viewed as a problem of sports injuries that cannot be ignored.
In the past, there have been vibration preventers that are mounted on the gut plane, bottom surface of the grip end or interior of the frame shaft in order to alleviate impact vibration of this type; however, no impact preventer that is detachably mounted on the outer circumferential portion of the frame shaft has been seen.
Typical vibrations that are the greatest in the frame shaft vibrations transmitted to the body when the ball is hit, and that have an effect on bodily injuries, include a two-node bending mode at approximately 120 Hz that is the fundamental mode characteristic of the frame shaft, as well as relatively higher-frequency vibrations such as a three-node bending mode at approximately 330 Hz, a two-node twisting mode at approximately 360 Hz, and a primary membrane vibrational mode of the strings at approximately 560 Hz. Besides these vibrations, there are numerous other vibrations up to high frequencies of approximately 2000 Hz, and it is known that respective characteristic non-vibrating parts or “nodes” are formed on the frame shaft in the main vibrational modes.
Here, the conditions of the “nodes” of the main vibrational modes are black and white boundary line portions as shown in FIG. 9 (cited from Yoshihiko Kawazoe: Rakketo no Kagaku [Racquet Science] II—: Gekkan Tenisu Jaanaru (Monthly Tennis Journal), 123, pp. 76–81 (1994.1)). The areas that are distant from these nodes are areas in which the vibration amplitude of the frame shaft is large, and are referred to as “bellies” of vibration. Especially in regard to the fundamental vibrational mode of the racquet, it has been found that the modes that make a large contribution to an unpleasant ball hitting sensation and injuries to the body are the two-node and three-node bending modes. Furthermore, it would appear that the mounting of vibration preventers in the areas forming the common “bellies” of both modes is an effective means of absorbing or attenuating vibration.
Furthermore, as is shown in FIG. 10, the respective implement portions that correspond to the racquet face portions (B1, B2, B3, B4), the front end portion (A) of the grip and the rear end (C) of the grip (in the vicinity of the grip end) constitute “bellies” where the vibration amplitude of the three-node bending mode that is generated in the case of relatively high-velocity center hitting is large. Accordingly, an improvement in the ball hitting sensation during center hitting would be expected when vibration preventers are mounted on these portions.
Furthermore, these implement portions are also common to the two-node bending mode that is generated mainly in low-velocity hitting and off-center hitting.
Accordingly, in light of the fact that the impact vibration caused by hitting of the ball is transmitted to the human body from the impact generating source via the frame shaft portion and handle portion, the present invention provides a sequential vibration preventer in which impact vibration is attenuated by mounting vibration preventers on the frame shaft that propagates such impact vibration, so that deleterious effects of such impact vibration on the body are prevented, and a grip vibration control structure using this sequential vibration preventer. The term “sequential vibration preventer” refers to a vibration preventer that has sequential or continuously lined up vibrators, and it is not a term indicating a special vibration mode for “sequential vibration.”